so... since I am going to look a a bus in the very near future - getting a quart from the bus and sending it to a DD dealer for analysis will not give me insight as to the condition of the engine since I don't have a series of analysis's over a period of time? Hope that made sense.

DD has a kit that includes everything you need to do the analysis except the postage. They're about $16 for everything. The postage from Alaska is only a couple of dollars.If you've never done this try it once and get your eyes opened. It cost less than 5 gallons of fuel.